1. Field of the Invention
The present invention relates to a technical field of cleaning a front surface of a substrate with a cleaning liquid while rotating the substrate.
2. Description of the Related Art
As exposure processing for forming a resist pattern on a substrate such as a semiconductor wafer, there is a known liquid-immersion exposure of performing exposure by making liquid exist on the front surface of the substrate. As a resist used for the liquid-immersion exposure, a high water repellent resist is used so as not to flow around to the peripheral edge and the rear surface of the substrate. In a developing treatment to be performed on the exposed substrate, a developing solution is supplied to the substrate to dissolve, for example, an exposed portion, and then a cleaning liquid such as pure water is supplied to the substrate while the substrate is being rotated to wash away a dissolution product from the front surface of the substrate. Concretely, a technique of scanning a cleaning liquid nozzle from a central portion of the substrate to the peripheral edge portion of the substrate while discharging the cleaning liquid from the cleaning liquid nozzle.
However, since a base film on which the resist is formed has low water repellency (small contact angle), the difference in contact angle between the exposed portion and an unexposed portion is large on the exposed substrate for which the resist having high water repellency (large contact angle) is used. Therefore, when the cleaning liquid is supplied after the developing solution is supplied, a phenomenon of so-called “liquid break” occurs and liquid drops become likely to remain on the front surface of the substrate. The liquid drops dry up to form residues, causing a reduction in yield of semiconductor devices.
As the base film, an anti-reflection film made of an organic material has been the mainstream, but an anti-reflection film made of an inorganic material having a smaller contact angle is discussed recently in which case the difference in contact angle between the exposed portion and the unexposed portion becomes larger and more likely to causes residues.
In the case of performing the above cleaning on the substrate having a large difference in contact angle between the exposed portion and the unexposed portion, it is effective to decrease the scan speed of the cleaning liquid nozzle, but the decrease causes a reduction in throughput of the apparatus. Since the coating and developing apparatus, in particular, is required to treat 200 or more substrates per hour in the market, a technique capable of reducing the residues while keeping a high throughput is desired.
As a cleaning method to solve the problem, Japanese Patent No. 4040074 describes a technique of discharging a cleaning liquid to a substrate and then discharging a nitrogen gas to a central portion of the substrate to form a core of a dry region, and thereafter, while moving the discharge position of the cleaning liquid to the outer side of the substrate, also moving the discharge position of the gas to spread the dry region to the outer side. In the technique, at the time when moving the discharge position of the gas, the moving speed of the gas nozzle is increased in a region on the peripheral edge side of the substrate. Japanese Patent Application Laid-open No. 2004-14972 (paragraph 0044) describes a technique of, when cleaning a substrate, changing the flow rate of a gas contained in a mixture of a liquid and the gas to be discharged toward the substrate as it gets closer to the peripheral edge portion of the substrate. Further, Japanese Patent No. 4350989 (FIG. 5, paragraphs 0050, 0053, 0057) describes a technique of increasing a spray angle of a gas to weaken the pressure of the gas as it gets closer to the peripheral edge of the substrate so as to uniformly dry. Further, Japanese Patent No. 5151629 (FIG. 4 to FIG. 6) describes a technique of discharging a gas toward a region of a substrate from one nozzle, and then discharging a gas toward the same region from another nozzle. In the future, it is predicted that a higher cleaning accuracy is required, and further improvements are demanded in order to reduce the residues.